This description relates to charged particle (e.g., proton or ion) radiation therapy.
The energy of a proton or ion beam for therapy needs to be high compared to the energy of an electron beam used in conventional radiotherapy. A proton beam, for example, that has a residual range of about 32 cm in water is considered adequate to treat any tumor target in the human population. When allowance is made for the reduction in residual range that results from scattering foils used to spread the beam, an initial proton beam energy of 250 MeV is needed to achieve the residual range of 32 cm.
Several kinds of particle accelerators can be used to produce a 250 MeV proton beam at a sufficient beam current (e.g., about 10 nA) for radiotherapy, including linear accelerators, synchrotrons, and cyclotrons.
The design of a proton or ion radiation therapy system for a clinical environment should take account of overall size, cost, and complexity. Available space is usually limited in crowded clinical environments. Lower cost allows more systems to be deployed to reach a broader patient population. Less complexity reduces operating costs and makes the system more reliable for routine clinical use.
Other considerations also bear on the design of such a therapy system. By configuring the system to apply the treatment to patients who are held in a stable, reproducible position (for example, lying supine on a flat table), the physician can more precisely relocate the intended target, relative to the patient's anatomy, at each treatment. Reliable reproduction of the patient's position for each treatment also can be aided using custom molds and braces fitted to the patient. With a patient in a stable, fixed position, the radiotherapy beam can be directed into the patient from a succession of angles, so that, over the course of the treatment, the radiation dose at the target is enhanced while the extraneous radiation dose is spread over non-target tissues.
Traditionally, an isocentric gantry is rotated around the supine patient to direct the radiation beam along successive paths that lie at a range of angles in a common vertical plane toward a single point (called an isocenter) within the patient. By rotating the table on which the patient lies around a vertical axis, the beam can be directed into the patient along different paths. Other techniques have been used to vary the position of the radiation source around the patient, including robotic manipulation. And other ways to move or reposition the patient have been used.
In high energy x-ray beam therapy, the x-ray beam may be directed toward the isocenter from an electron linear accelerator mounted on the gantry or robotic arm.
In typical proton beam therapy, the circular particle accelerator that produces the beam is too large to mount on the gantry. Instead, the accelerator is mounted in a fixed position and the particle beam is redirected through a rotating gantry using magnetic beam steering elements. Blosser has proposed to mount an accelerator on the side of the gantry near the horizontal axis of rotation.